Aid convoys have delivered long-awaited food, medicine and other supplies to three besieged Syrian communities cut off for months by fighting amid reports that hundreds are on the brink of death.

Reports of starvation and images of emaciated children have raised global concerns and underscored the urgency for new peace talks that the United Nations is hoping to host in Geneva on January 25.

UN humanitarian chief Stephen O'Brien said about 400 people in the hospital in the besieged mountain village of Madaya, near Damascus, must be evacuated immediately to receive life-saving medical attention. After briefing the UN Security Council he said they needed treatment for medical complications, severe malnourishment and starvation.

This must be done as soon as possible "or they are in grave peril of losing their lives", Mr O'Brien said, adding that efforts would be made to get ambulances to Madaya on Tuesday to evacuate the 400 people if safe passage could be assured.

The UN says 4.5 million Syrians are living in besieged or hard-to-reach areas and desperately need humanitarian aid, with civilians prevented from leaving and aid workers blocked from bringing in food, medicine, fuel and other essentials.

It will take several days to distribute the aid in Madaya and the Shiite villages of Foua and Kfarya in northern Syria. The supplies are probably enough to last for a month, aid agencies say.

"It's really heartbreaking to see the situation of the people," said Red Cross spokesman Pawel Krzysiek, who oversaw the distribution in Madaya. "A while ago I was just approached by a little girl and her first question was, 'Did you bring food?'."

Mr Krzysiek said he saw a lot of people on the street, "some of them smiling to us and waving to us, but many just simply too weak".

Sajjad Malik, a representative of the UN High Commissioner for Refugees, who took part in the aid operation, said: "It's cold and raining but there is excitement because we are here with some food and blankets."

Peter Wilson, Britain's deputy UN ambassador, said it was "good news that those convoys are getting through, although it's little and it's late".

"It's important to remember that Madaya represents only 10% of those who are under siege and 1% of those who need aid in Syria," he added.

The mercy mission marked a small, positive development in a bitter conflict now in its fifth year that has killed a quarter of a million people, displaced millions of others and left the country in ruins.

"This has to be just a start," said New Zealand's UN ambassador Gerard van Bohemen, who, along with Spain, called for the security council meeting. "It can't be just a one-off situation. Humanitarian access cannot be held hostage to politics."

Rebels opposed to President Bashar Assad are in control of Madaya and government troops and fighters from the Lebanese militant group Hezbollah have surrounded the town. Opposition activists and aid groups have reported several deaths from starvation in recent weeks.

But Syria's UN ambassador Bashar Ja'afari denied anyone was starving in Madaya and blamed Arab television especially "for fabricating these allegations and lies".

Speaking at the UN's headquarters in New York, he blamed "armed terrorist groups" for stealing humanitarian aid and reselling it at prohibitive prices.

"The Syrian government is not and will not exert any policy of starvation against its own people," Mr Ja'afari said.

But Mr O'Brien said all the evidence showed there had been very severe malnourishment, severe food shortages, and reports of people "who are either starving or indeed have starved and died".

An Associated Press crew saw the first three trucks cross into Madaya, although journalists were not allowed to accompany the aid workers. At the town's entrance, several civilians - including five children shivering against the cold - said they were waiting to be taken out.

"I want out. There is nothing in Madaya, no water, no electricity, no fuel and no food," said teacher Safiya Ghosn.

Simultaneously, convoys began entering Foua and Kfarya, which are both under siege by rebel groups hundreds of miles to the north.

Tales of hunger and hardship have emerged from those inside all three communities. Pro-government fighters recently evacuated from inside Foua and Kfarya have said some people are eating grass to survive and Madaya residents have reported living off soup made of leaves and salt water.

Madaya has attracted particular attention in recent days because of reports of deaths and images in social media of severely malnourished residents. The aid operation, which is being facilitated by the UN, was agreed last week.

The aid group Doctors Without Borders said 23 people had died of starvation at a health centre it supports in Madaya since December 1, including six infants and five people over 60.

Nearly 42,000 people in the town were at risk from hunger, said Yacoub El Hillo, the UN's resident and humanitarian co-ordinator in Syria.

Hezbollah's Al-Manar TV channel showed a group of people waiting for the convoys at Madaya's main entrance. In interviews, they accused rebel fighters inside of hoarding humanitarian assistance that entered the town in October and selling the supplies to people at exorbitant prices.

Ms Ghosn also blamed rebels in Madaya, saying: "Their depots are full while we go hungry. We have to humiliate ourselves to go to them and beg for food."

A group of eight major international aid groups, including CARE International, Oxfam, and Save the Children, welcomed the aid convoy but warned that a one-time delivery would not save starving people.

"Only a complete end to the six-month-old siege and guarantees for sustained aid deliveries alongside humanitarian services will alleviate the crisis in these areas," a statement by the group said.